The present disclosure relates to a vehicle-mounted signal generation circuit and a vehicle-mounted power supply device.
Conventionally, a voltage conversion device for converting a voltage by driving a switching element with PWM (Pulse Width Modulation) signals has been widely used. In this PWM control voltage conversion device, for example, a voltage command value is calculated based on a target value of a voltage, a value according to the calculated voltage command value is set in a PWM signal generation unit, and thus PWM signals having a duty ratio according to the set value is generated. In this manner, by changing the duty ratio of the PWM signals that drive the switching element in accordance with the target value of the voltage, an output voltage according to the target value of the voltage value can be acquired.
Here, if a minimum unit of a value (that is, a minimum increment) that can be set to a PWM signal generation unit is relatively large, the duty ratio of the PWM signals cannot be smoothly changed with respect to changes of the target value, and thus the output voltage changes stepwise. Also, for example, if the target value to be set in the PWM signal generation unit is calculated as an operation amount of the PWM control, when the minimum unit of the value that can be set is larger than the minimum unit of the target value, the duty ratio of the PWM signals cannot be smoothly changed with respect to the change of the target value of the voltage and load variation, and thus an error occurs in the output voltage.
In this regard, JP 3-98470A discloses a PWM inverter in which, when an on/off-time of a PWM signal is computed every PWM control period, the on/off-time is calculated by performing a computation while rounding down the remainder of a division using a voltage command value as a dividend, and a PWM pulse is output based on the calculation result. The remainder that occurs in the above computation corresponds to a voltage command value that is not reflected in the on/off-time and rounded down.
In this PWM inverter, the remainder that has been rounded down is sequentially added to the voltage command value in the computation in the next and subsequent cycles. Thus, the remainder that was not reflected in the on/off-time in the previous computation is reflected in the new on/off-time at the time of the next computation, the remainder at this time is further reflected in the next computation, and this computation is repeated. As a result, the average value of the on/off-time that is to be set for the PWM signal generation unit can be brought closer to a target on/off-time that is to be originally set. That is, the minimum increment of the value set for the PWM signal generation unit can be set smaller than the actual increment in average.
On the other hand, another problem related to the PWM control is a problem of noise caused by fixing the PWM cycle. Frequency spread PWM control has been proposed as a method for reducing this kind of noise, in this frequency spread PWM control, occurrence of noise is suppressed by randomly changing the PWM cycle. As a technique relating to this frequency spread PWM control, there is a technique as disclosed in JP 2010-130850A, for example.